A marrow aspiration device is designed to take a liquid sample of bone marrow tissue. Typically, a device that is utilized to aspirate samples from bone consists of a hollow cannula that surrounds a stylet. The stylet includes a sharp distal tip which extends outwardly from a distal opening of the cannula when the stylet is placed inside the cannula. The combined cannula and stylet are used to penetrate through the outer layer of the bone, called the cortex, which is considerably harder than the inner layer, called trabecular bone, and the tissue within the trabecular bone that is sampled, referred to as the marrow. Once the stylet and cannula have penetrated the cortex, the stylet is removed which exposes a connector in the handle that is attached to the cannula. A syringe is attached to the connector, negative pressure is applied by pulling on the plunger handle of the syringe, and a sample of tissue from the bone space is retrieved into the syringe.
Blood and marrow have different viscosities and cellular characteristics. Blood has a lower viscosity than marrow. The goal of a good aspirate is to retrieve as much marrow and as little blood as possible. However, since blood has a lower viscosity, it preferentially flows in response to the negative pressure created by the syringe. Inserting a cannula with a sharp stylet into the bone space breaks the inner trabecular bone as a path is created equal to the outer diameter of the cannula. The sharp stylet protrudes from the end of the cannula. Removing the sharp stylet creates a void equal to the portion of the stylet that extended beyond the distal end of the cannula. Upon removal of the stylet, blood fills that void as the pressure created by the stylet, which was preventing greater bleeding, is removed. This blood is now in contact with the opening at the end of the cannula. Applying negative pressure will create the greatest pressure against the opening which is in contact with the lower viscous tissue. Therefore, a typical aspiration device will preferentially draw blood over marrow even if the cannula has small side apertures. Pulling the cannula back while aspirating creates a longer channel that fills with blood and exacerbates the problem of preferentially drawing blood. Since there are only a finite number of marrow cells in any given location of the trabecular bone space, drawing all of a sample from a single location by not moving the cannula during the aspiration process gives a rapidly diminishing return on the number of cells after only drawing a small sample (e.g., 1 mL).
Therefore, there is a need for a system that aspirates marrow in a manner that minimizes the level of peripheral blood.